Various industrial production processes of acetic acid have been known. Among others, an industrially excellent process includes a process which comprises continuously allowing methanol to react with carbon monoxide with the use of a rhodium catalyst and methyl iodide in the presence of water to give acetic acid. Moreover, recently improvement in reaction conditions and catalysts was investigated, and an industrial process for producing acetic acid with a highly efficient production has been developed by addition of a catalyst stabilizer (such as an iodide salt) and the reaction under a low water content condition compared with the conventional condition. This process enables by-products including carbon dioxide or propionic acid to be reduced because of lowered water content in the reaction mixture. In the reaction mixture, however, there are by-products (impurities) in minor quantities other than these components, for example, a carbonyl compound (e.g., acetaldehyde, butyraldehyde, crotonaldehyde, 2-ethylcrotonaldehyde, and an aldol condensation product thereof), an organic iodide (e.g., an alkyl iodide other than methyl iodide, such as ethyl iodide, butyl iodide, or hexyl iodide). As the production of acetic acid increases, the production of the impurities increases, which deteriorates the quality of acetic acid. For example, in a quality test by which the very minute amounts of the reducing impurities present in acetic acid are checked, which is called a permanganate-reducing substance test (permanganate time), impurities having very slight concentrations, which are hard to quantitatively determine even with today's highly advanced instrumental analysis, can be detected, and these impurities lead to deterioration of product quality. Moreover, some of these impurities exert a bad influence in connection with use of acetic acid. For example, in a process for producing vinyl acetate from ethylene and acetic acid, it is known that the impurities deteriorate a palladium-series catalyst used in the process.
Unfortunately, the carbonyl compound or the alkyl iodide is hard to remove sufficiently by ordinary means, such as distillation, because the boiling point of the carbonyl compound or the alkyl iodide is close to a boiling point of an iodide catalyst accelerator, or other reasons.
Thus, for example, the treatment of crude acetic acid containing these minute impurities with ozone or an oxidizing agent has been developed. Because the treatment has the limitation on the concentrations of impurities to be treated, the carbonyl compound and the organic iodide cannot be removed efficiently.
Meanwhile, in a continuous reaction process, removal of a carbonyl compound in a process circulation liquid is also being attempted. For example, Japanese Patent Application Laid-Open Publication No. 4-266843 (JP-4-266843A, Patent Document 1) discloses a process for removing a carbonyl impurity, which comprises allowing a methyl iodide stream to be recycled to a carbonylation reactor to contact with an amino compound that is allowed to react with a carbonyl impurity to form a water-soluble nitrogenous derivative, separating the resulting organic methyl iodide phase from the resulting aqueous derivative phase, and distilling the methyl iodide phase. The concentration of the carbonyl impurity in the organic stream to be recycled to the carbonylation reactor is still high, and it is difficult to remove the carbonyl impurity sufficiently. Moreover, the process described in this patent document requires removal of the nitrogenous compound.
Japanese Patent Application Laid-Open Publication No. 8-67650 (JP-8-67650A, Patent Document 2) discloses a process for producing high-purity acetic acid, comprising allowing methanol to continuously react with carbon monoxide in the presence of a rhodium catalyst, an iodide salt, and methyl iodide, wherein the reaction is carried out by removing acetaldehyde from a process liquid being circulated into a reactor to maintain the acetaldehyde concentration in the reaction mixture at 400 ppm or lower. This patent document forcuses attention on that most of the impurities are formed in the reaction system and the formation of such impurities is due to the by-product acetaldehyde produced in the reaction system. Therefore, the concentration of acetaldehyde in the reaction system is reduced to decrease the carbonyl compound or the organic iodide, and, as s result, high-purity acetic acid can be obtained.
In addition, this patent document discloses, relating to a process for producing acetic acid while removing acetaldehyde, that the process comprises separating the reaction mixture into a volatile phase containing acetic acid, methyl acetate and methyl iodide and a low-volatile phase containing the rhodium catalyst, distilling the volatile phase to obtain a product mixture containing acetic acid and an overhead containing methyl acetate and methyl iodide, and recirculating the resulting overhead into the reactor, wherein the overhead or a carbonyl impurity (particularly acetaldehyde) condensate thereof is allowed to contact with water to separate an organic phase containing methyl acetate and methyl iodide and an aqueous phase containing the carbonyl impurity, and the organic phase is recirculated into the reactor. Moreover, this document discloses that a concrete method for obtaining methyl iodide from the carbonyl impurity condensate preferably includes a method which comprises separating an acetaldehyde liquid containing methyl iodide from the process liquid by distillation, and selectively extracting the resulting acetaldehyde liquid with water.
Incidentally, this document does not disclose that the flow rate of the overhead is controlled in recirculating the overhead into the reactor.